The present invention relates to organic light-emitting diode devices and in particular to high contrast devices and methods for making such devices.
Organic electroluminescent (OEL) device, alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; it is operable with a low driving voltage of the order of a few volts and viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of thin films of small-molecule organic materials sandwiched between a hole-injecting and an electron-injecting layers. Tang et al in U.S. Pat. Nos. 4,769,292 and 4,885,211 has disclosed such a structure. The most common electroluminescent (EL) medium comprises a bi-layer structure of a hole-transport (HTL) layer and an emissive and electron-transport layer (EML/ETL), typically of the order of a few tens of nanometer thick for each layer. The anode, or the hole-injecting electrode, is usually an optically transparent indium tin oxide (ITO) glass, which also serves as the substrate for the OLED. A low-work function metallic layer is selected as cathode, or electron-injecting electrode for the device. The device emits visible light in response to a potential difference applied across the EL medium. The light is emitted in all directions. A fraction of light that strikes the cathode surface is reflected and directed toward the anode. This adds to the intensity of light passed through the ITO glass. High reflectivity cathode is thus preferred as it helps to enhance the brightness of emission. However, highly reflective cathode also reflects significant amount of ambient light that enters the device through ITO glass. At high illumination level the reflected ambient light can overpower the EL emission causing an apparent degradation of emitted light. To an observer the emitted light visually perceived washed out. Increasing the ambient illumination increases the washed out appearance and lowers the visual contrast. In many applications, particularly in outdoors or in brightly illuminated room, visual contrast may be more important than the intensity of emission. In quite a few applications it is required that the displays can be viewed rather easily under a variety of ambients ranging from total darkness to full sunlight. One of the ways of achieving an enhancement of visual contrast is by reducing the reflection from cathode.
A well-known approach to reduce glare is to use polarizers, particularly circular polarizers on the outside surface of transparent substrate. However, this requires an additional bonding step incompatible to the OLED fabrication process adding significant cost. Furthermore, polarizers significantly reduce the emitted light intensity.
Another approach disclosed by Luxell Technologies (Journal of Military and Aerospace Electronics, Vol 9, No 6, June 1998) yielded inorganic light-emitting (EL) displays with reduced glare and enhanced sunlight readability. An optically tuned interference structure is interposed between the shiny back electrode and phosphor material in the EL stack. This interference structure termed as the xe2x80x9csunshine legible black layerxe2x80x9d consists of vacuum deposited layers of absorbing and dielectric materials. This technology produced displays with 14% reflectance.
In another approach an n-type oxygen-deficient semiconductor layer of ZnO1xe2x88x92x was used as the interference tuning layer and laid over an ultra thin LiF/Al bi-layer cathode. The structure of the device was
ITO/NPB/Alq/LiF/Al/85 nm ZnO1xe2x88x92x/Al. This device also exhibited greatly reduced reflectivity relative to a standard device.
Often, the reflection reducing materials require that for minimization of reflectivity, the thickness of the interference tuning layers be of the order of 100 nm. Fabrication of these types of structures necessarily requires long deposition time. It is also clear that to prepare the interference-tuning layer with reproducible and predictable optical properties the deposition parameters for these types of materials have to be tightly controlled. Furthermore, these devices may also be potentially unstable in the long term. Thermodynamic data suggest that Al can reduce ZnO1xe2x88x92. The Al2O3 that is likely to form at the ZnO1xe2x88x92/Al interface can degrade the operating voltage of the device. No stability data of those devices were reported.
Renault et al (O. Renault, O. V. Salata, M M. Chells, P. J. Dobson, V. Christou) in Thin Solid Films 379 (2000) 195-198 describe a low reflectivity multilayer cathode that shows promise for contrast improvements in OLEDs. The device structure incorporates a light absorbing carbon film between a thin semitransparent electron-injecting layer of Mg and a top Al layer. The reflectivity of low reflectivity cathode was 58% at 550 nm. The standard cathode comprised of Al:Mg (10:1) exhibited a reflectivity of xcx9c100% at 550 nm. The current-voltage properties of the two devices were almost identical. Although a lower reflectivity is needed for contrast improvements, the contrast also depends on the emitted light intensity and ambient illumination. Their reported values of EL intensities at 22 eV for standard and low reflectivity devices were 130 Cd/m2 and 68 Cd/m2, respectively. Following their definition of contrast and using the reported reflectivity and luminance values, no increase of contrast over that of the standard
It is therefore an object of the present invention to provide a novel OLED device having higher display contrast arising from reduced reflection from the reflecting layer.
It is another object of the present invention to provide an improved OLED device having higher display contrast arising from reduced reflection from the reflecting layer wherein an interlayer is used in the device structure.
It is another an object of the present invention to provide an improved OLED device having higher display contrast arising from reduced reflection from the reflecting layer wherein in the interlayer material is a semi-metal, metal, or intermetallic compound or alloys thereof.
It is further an object of the present invention to provide an improved OLED device having higher display contrast arising from reduced reflection from the reflecting layer wherein in the interlayer is very thin.
The objects are achieved by an OLED device for providing increased contrast, comprising:
a) a transparent substrate;
b) an anode formed of a transparent conductive material over the substrate;
c) an emissive layer having an electroluminescent material provided over the anode layer;
d) an ultrathin low absorption electron-injecting layer laid over the electron-transport layer;
e) an interlayer including a semi-metal, metal, metal alloy or an intermetallic compound disposed over the ultrathin low absorption electron-injecting layer;
f) a layer of reflective material provided over the interlayer; and
g) wherein the interlayer thickness is selected to cause an increase in contrast.
This object is further achieved by an OLED device for providing increased contrast, comprising:
a) a transparent substrate;
b) an anode formed of a transparent conductive material over the substrate;
c) an emissive layer formed over the substrate and including an electroluminescent material;
d) an interlayer provided over the emissive layer including a semi-metal, metal, metal alloy or an intermetallic compound;
e) a layer of reflective material provided over the interlayer; and
f) wherein the reflective layer is doped with an electron-injecting dopant which migrates sufficiently to provide an electron-injecting connection between the reflecting layer and the emissive layer and wherein the interlayer thickness is selected to cause an increase in contrast.
An advantage of the present invention is that an OLED device has significantly reduced ambient-light reflection from the device and substantially increased contrast compared to devices having no interlayer but otherwise similar in structure.
Another advantage of the present invention is that OLED devices of the present invention have a very thin and conducting interlayer.